An internal error occured while processing mixdown

welcome to the forum.

Just a few things you need to know.
SONAR doesn't play mp3 files. You can import an mp3 files at which point SONAR converts it to .wav files. SONAR works on .wav files only but it can import a vast array of different types.

To export to mp3 you either must
1. have an external mp3 converter configured for use in SONAR or
2. have bought the Cakewalk Mp3 activation code or
3. you are still within the 30 day trial of the demo version that comes with a copy of SONAR.

So what we need to know is what process you are following to come up with this unspecified error.

When you export from SONAR you need to select a variety of options in the Export Audio dialog box to successfully get what you want. It would benefit you to Open SONAR press F1 to open the help file and in the Index type Export Audio Dialog - press enter. It will give you an explanation of all the possible options available when you want to export audio.